One example of the surface acoustic wave device of the unidirectional type is desclosed in the reference entitled "a flat-amplitude surface acoustic wave filter in corporating a group-based unidirectional interdigital electrode array" by Toshiyasu Meguro et al, in December, 1976 in "a Collection of Lectures 1-5-14" of Japan Acoustic Academy. Another example of the undirectional SAW device is desclosed in the copending U.S. patent aplication (Ser. No. 869,979) filed on June 2, 1986 and now U.S. Pat. No. 4,689,586 and in DE-OS No. 3,618,913.
In such unidirectional SAW devices the insertion loss and the distortion in the frequency characteristic can be further reduced over the wide band width. However it is difficult to make such a unidirectional device exhibit asymmetrical amplitude frequency characteristics. In particular, none of the conventional SAW devices suggests a unidirectional SAW device which exhibit such asymmetrical characteristics with a simple external circuit, which may be a single phase shift circuit.